Name Class Date
9.2 The Process of Cellular Respiration
Lesson Objectives
Describe what happens during glycolysis.
Describe what happens during the Krebs cycle.
Explain how high-energy electrons are used by the electron transport chain.
Identify how much ATP cellular respiration generates.
Lesson Summary
Glycolysis The word glycolysis literally means “sugar-breaking.” The end result
is 2 molecules of a 3-carbon molecule called pyruvic acid.
2 ATP molecules are used at the start of glycolysis to get the process started.
High-energy electrons are passed to the electron carrier NAD+, forming two molecules of
NADH.
4 ATP are synthesized during glycolysis for a net gain of 2 ATP.
The Krebs Cycle The second stage of cellular respiration is the Krebs cycle, which
operates only when oxygen is available. The Krebs cycle is a series of energy-extracting
reactions.
Pyruvic acid produced by glycolysis enters mitochondria. In the innermost compartment
of a mitochondrion, or the matrix, pyruvic acid molecules are broken down into carbon
dioxide and acetyl-CoA molecules.
Acetyl-CoA combines with a 4-carbon compound, producing a 6-carbon molecule—citric
acid. Energy released by the breaking and rearranging of carbon bonds is captured in
ATP, NADH, and FADH2.
The Krebs cycle produces four types of products:
high-energy electron carriers (NADH and FADH2)
carbon dioxide
2 ATP molecules (per glucose molecule)
the 4-carbon molecule needed to start the cycle again
Electron Transport and ATP Synthesis The electron transport chain uses
the high-energy electrons from glycolysis and the Krebs cycle to convert ADP into ATP.
The electron carriers produced during glycolysis and the Krebs cycle bring high-energy
electrons to the electron transport chain. Oxygen is the final electron acceptor.
The passing of electrons through the electron transport chain causes H+ ions to build up in
the intermembrane space, making it positively charged relative to the matrix.
The charge difference across the membrane forces H+ ions through channels in enzymes
known as ATP synthases. As the ATP synthases spin, a phosphate group is added to
ADP, generating ATP.
The Totals Together, glycolysis, the Krebs cycle, and the electron transport chain
generate about 36 molecules of ATP per molecule of glucose.
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